SILVER IN PHOTODOPED ARSENIC SULFIDE FILMS : AN EXAFS STUDY

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SILVER IN PHOTODOPED ARSENIC SULFIDE FILMS : AN EXAFS STUDY

A. Steel, A. Firth, A. Owen, P. Ewen, G. Greaves

To cite this version:

A. Steel, A. Firth, A. Owen, P. Ewen, G. Greaves. SILVER IN PHOTODOPED ARSENIC SULFIDE FILMS : AN EXAFS STUDY. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-395-C8-398.

�10.1051/jphyscol:1986878�. �jpa-00226200�

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SILVER I N PHOTODOPED ARSENIC SULFIDE FILMS : AN EXAFS STUDY

A.T. STEEL", A.P. FIRTH"', A.E. OWEN"', P. EWEN'" and G. N. GREAVES'

* ~ e p a r t m e n t of Chemistry, Manchester University, GB-Manchester M13 9PL, Great-Britain

Department Elec. Engineering, Edinburgh University, GB-Edinburgh EH9 3JL, Great-Britain

*Daresbury Laboratory, GB-Warrington, W A 4 4AD, Great-Britain

RBsume'

Nous avons mesur6s l e K-edge EXAFS de I ' a r g e n t p o u r u n nombre de f

f

lms d u s u Cfure d ' a r s e n i c photodopk e t determ

I

nes l a d I s t a n c e de c o o r d

i

n a t i o n d'argent 2 s o u f r e . C 'est t r & s semb Lab l e 2 l a v e r r e Ag2,Asj,&, comme l e XANES. L ' o n a observ; i a sGparat i on des .phases dans quelques ff lms p a r l'apparence d'une seconde couche dans l a t r a n s f o r m a t i o n de F o u r i e r .

Abstac?

We have measured t h e s r l v e r k-edge EXAFS f o r a number o f photodoped a r s e n

ⁱ

c s u l p h

I

de f

i

lms and d e t erm

I

ned t h e s i l v e r - s u l p h u r c o o r d

i

n a t

i

on d i stance. Th

i s

i s v e r y s i mi Car t o t h e Ag15A%5S50g la s s d as

i

s t h e near-edge r e g i o n - P h a s e s e p a r a t i o n has been observed i n some f i l m s by t h e

appearance o f a second s h e l l i n t h e F o u r i e r t r a n s f o r m .

INTRODUCTION

L

i

g h t -

i

nduced d i sso l u t i o n o f s

i

l v e r (and copper)

¹

n t o cha lcogen i de g l a s s e s has been s t u d i e d f o r some t i m e and has p o s s i b l e a p p l i c e t f o n s

i n

sub-micron

l i

chography ( 1 ) as a r e s u

!t

o f t h e v e r y smal l amount o f

L a t e r a l d i f f u s i o n . R a m a n s p e c t r o s c o p y o f photo-doped

f i i m s

based on arsenr c s u lphr de composr

t i

ons As, S,-, (x=20,30,40) ( 2 , 3 ) has i nd

i

c a t e d t h a t when x=30 t h e s t r u c t u r e i s v e r y s i m i l a r t o a b u l k g i a s s and t h a t t h e mater

i

a 1 c o n s f s t s o f a homogeneous, unr form phase. I n t h e x=40 a r s e n i c - r i c h f i l m a phase-separated m a t e r i a l c o n t a i n i n g amorphous a r s e n r c and an amorphous Ag-As-S phase.For x=20 ! n ! t i a l f o r m a t i o n o f a s i l v e r s u l p h i d e phase i s proposed.An u n d e r s t a n d i n g o f i h e

c o o r d i n a t i o n environment adopted b y t h e s t Cver An t h e photodoped f l lms may h e l p d e t e r m i n e t h e mechanism o f d o p i n g and g i v e some i n d i c a t i o n s about phase-separation.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986878

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JOURNAL DE _PHYSIQUE

SAMPLE PREPARATION

The f i

l m s

a r e p r e p a r e d by e v a p o r a t i ng approx i mate l y 10008 o f s

i

l v e r o n t o a m y l a r sheet f o l l o w e d by 5000-60001 o f a r s e n i c s u l p h i d e . T h i s

i s

t h e n exposed (from t h e c h a l c o g e n i d e s i d e ) t o a 2001 Hg lamp w i t h a f i l t e r so t h a t wavelengths i n t h e r a n g e 350-500nm r e a c h t h e sample.The exposure

t i

me i s approx

I

mate l y 30m i n s a t wh i c h

t i

me t h e samp i e cannot be e t c h e d w i t h a d i l u t e a l k a l

i

ne s o l u t ion. The f

i l m s

d e r i v e d from u s i n g o n l y 4000X o f a r s e n i c s u l p h i d e showed a s l i g h t

m i

l k i n e s s and

i t

i s t h o u g h t t h a t t h i s i n d i c a t e s i n c o m p l e t e d o p i n g o r phase-separation.The EXAFS o f t h e s e samples appear t o show e v i d e n c e o f Ag--A9 b o n d i n g from s

i

l v e r met a 1. The o t h e r f

I

lms were complete l y clear..

EXAFS

The EXAFS a t t h e

6 1

l v e r k-edge have been r e c o r d e d on t h e w ~ g g l e r beam- l i n e o f t h e SRS a t s t a t ron 9.2. S u ~ t a b l e c r y s t a l l i n e model compounds f o r t e s t

I

n g t h e ca l c u l a t e d phase-sh i f t s and compar

I

son w

1

t h t h e f

I

Lms, Ag2S ,acanthite,Ag3AsS3.proust ~ t e , A g A s S ~ , s m ~ t h i t e , t o g e t h e r w ~ t h a b u l k g Lass o f compos i

t I

on Ag25~skS3, have been used. F T g. 1 shows t h e qua l

i t

y o f raw d a t a w

i t

h t h e l e a s t -squares f

I t,

and t h e f o u r

I e r - f I l t

e r e d f

I

r s t s h e l l and f ~ t f o r t h e f i l m d e r i v e d from t h e c o m p o s t t i o n A s S and o f t h r c k n e s s r a t i o 5 . 5 ~ l . A l L t h e d i s t a n c e s , e t c shown i n t a b l e 1 a r e d e r ~ v e d from f o u r

i

e r - f

I

l t e r e d data.

FIG.

1 (a)

3R(A?

1 2 5

g w e

i

cJhted EXAFS above t h e

s I

l v e r K-edge and FT f o r t h e As3Sm (5.51 1

) f i lm.

(a) Raw d a t a (b) f

i

r s t -she 11 f o u r

i

e r - f

i l t

e r e d data. -Expt. - - - i h e o r y

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2 k - w e l o h t e d EXAFS above t h e s ~ l v e r K-edge aqd FT f o r t h e As

?U

S 70 4

^'

0 : l f

I

Lm. -Expt. ---Theory

3. k - w e t g h t e d EXAFS above t h e s i l v e r K-edge and FT f o r a gery t h i c k

I

n h ~ m o ~ e n e o u s f

i lm.

-Expt . ^---Theory

h i L t h e f i Lms h a d bas

i

ca I i y t h e seine EXAFS

I .

e. a s i n g l e f r e q u e n c y from a f

i

r s t - s h e L l o f su Lphur atoms. I n t w o cases, however, a s e c o n d - s h e l l i s observed. When

a

v e r y t h i c k f

i l m

(approx. 1 m i c r o n s

i

l v e r ) was p r e p a r e d t h e second s h e l l ( f i g . 3 ) a r i s e s from Ag--Ag o f p h a s e - s e p a r a t e d s i l v e r m e t a l . I n t h e As3,,S7,,fi l m ( 4 : I t h i c k n e s s ) ( f l g . 2 ) t h e second s h e l l I S sma l l e r and i t s i n t e r p r e t a t i o n i s Less c l e a r . There i s

a

s irni l a r peak

i n A g Z S a r

i s I

ng from Ag--AS non-bond

i

ng

i

n t e r a c t i ons

a t

approx. 3. OX,

b u t t h e d i s t a n c e i s i n c o r r e c t . The f o r m e r e x p l a n a t i o n i s f a v o u r e d b u t more work w i t h p u r p o s e l y - i n d u c e d i n h o m o g e n e i t y i s r e q u ~ r e d .

The s i l v e r - s u l p h u r c o o r - d i n a t i o n w h i c h i s

l i

k e l y t o be qu i t e

d

i sordered, i n

i

n e w

i t

h t h e mode 1 compound, h a s been f

i

t

e d i n k-space

u s

i

n g one she l L o f s u l p h u r atoms, and

a

1 l o w i n g t h e c o o r d

i

n a t i o n number and Debye-Walker f a c t o r t o vary.The models have b e e n t r e a t e d s

i m i

t a r l y f o r c o m p a r i s o n purposes. F o r example

i t

i s p o s s i b l e t o use a l l f i v e c o o r d i n a t i n g d i s t a n c e s i n A g z S b u t t h e f i t i s l i t t l e improved.

Any conc l u s i ons, t h e r e f o r e , based on t h e numbers and Debye-Wa l l e r f a c t o r s

a t t h i s s t age would be r a t h e r specu t a t

i

ve arid d i s c u s s

i

on i s c o n f i n e d

t o t h e bond d i s t a n c e and more g e n e r a l a s p e c t s .

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JOURNAL DE PHYSIQUE

TABLE!

Conlp. o f s t a r t i ng As-S rnater

ⁱ

a 1

A ~ z S Ag,AsS, AgAsS,

Ag25As,SS50 (g Lass1 As2275

As255

,5

A sJCSTo A s3,S Asyo"

do

Thickness r a t

i

o o f As-S/Ag

The main s

I

l v e r - s u l p h u r i n . t h e f

i ! m s

i s a t 2.46x which

i s

between that; f o r p r o u s t

i

t e and s r n i t h i t e b u t s i m i l a r t o t h a t f o r Ag2S. The Laci:

o f second s h e ( l a s seen i n t h e l a t t e r r u l e s t h i s o u t as a mode 1 f o r t h e c o o r d i n a t io:i. However t h e r e i s gooed

s i rn i

i a r i - t y between t h e f

i i m s

and t h e bu i k g l a s s I n which no second s h e ( 1

I S

observed. To r e i n f o r c e t h i s a compprison o f t h e XANES

^{I S}

shown and t h a t o f t h e f i l m s i s aimost i d e n t i c a l t o t h a t o f t h e g l a s s .

CONCLUSION

It i s c l e a r t h a t phase-separat i o n and inhomogene i t y can b e a p r o b lem

i n

t h e s e p h o t odoped f

i Lms,

and y e t most o f t h e f

i L m s w e

have s t u d i e d have been reasonab Ly homogeneous. The env I ronment o f t h e s

i

i v e r 1s c l o s e s t t o t h a t found

i n

t h e b u l k g l a s s . An i mproved u n d e r s t a n d i n g

o c

t h e g l a s s e s wi L i be o f g r e a t importance

i n

d e t e r m i n i n g t h e s t r u c t u r e of t h e f

i

lms. An i m p o r t a n t quest i o n t h a t may be answered b y u s i n g EXAFS

i s whether t h e g l a s s - f o r m i n g c o m p o s i t i o n r e g i o n i s s i g n i f i c a n t l y extended i n t h e s e t h i n f i ims.

eV

0

10

20

30

40

50

I I I

- - - .

...

AgAsS,

U

AgS

1:

I ^XhNES ^compar

ⁱ

^son ^{o f} ^{t h e} ^f

ⁱ

^lrns and t h e mode 1 compounds.

REFERENCES

1 ) M. Jane

I

9 t h . I n t . Conf. on Amorphous and L i q u

^I

d Sern i conductors, Grenob Le, France, J u l y 198i

2) A. P. F i r t h , P. S. Ewen and A. E. Owen S t r u c t . o f Non-Cryst. Mater

i

a 1s (Ed P. H. Gas ke I L ) Pub 1. Tay !or and F r a n c

i s

1982

3 ) A. P. F i r t h , A. E. Gwen and ?..J. Ewen Journa 1 de Phys

ⁱ